1. Field of the Invention
The present invention relates to a motor control circuit, and particularly to a control circuit for protecting the motor providing the motive power in an electrically-powered motor vehicle.
2. Description of the Prior Art
The development of electrically-powered motor vehicles, particularly electric cars, has been pursued in recent years as one means of addressing environmental problems. The power source for these vehicles is either an AC or DC motor. In particular, high efficiency synchronous motors using a permanent magnet for the rotor are being actively developed for electric cars.
While synchronous motors of this type are widely used in factory automation applications in industry, they are also suited for use in electric cars, and the basic control method is essentially the same.
In the factory automation field, overcurrent control by the power element controlling the motor immediately outputs a stop signal when an overcurrent is detected, and completely stops the motor. If this same control principle is applied in an electric car, however, the vehicle may suddenly and unexpectedly stop, creating an obvious safety hazard that could result in a rear-end collision with the vehicle following behind.
A control method whereby the current supply is completely and immediately cut off when the motor current exceeds a .predetermined threshold value may thus cause the vehicle to suddenly stop, creating a life-threatening situation and obvious safety problem. On the other hand, failure to interrupt the current supply when an overcurrent to the motor is detected may result in serious damage to the motor, and is also not desirable.
In addition to damaging the power element and/or motor, however, an overcurrent can also cause a car fire. An electric car control circuit whereby these hazards can be reliably prevented and the vehicle can safely avoid hazardous situations caused by problems related to the motor current supply is therefore necessary.
Therefore, an object of the present invention is to provide a control circuit providing for improved safety in electric car operation by temporarily interrupting the motor power supply as a means of eliminating an overcurrent supply to the motor when a motor overcurrent is detected, and completely shutting off the motor power supply if a value integrating the overcurrent detection time exceeds a predetermined time limit.